Conformation of cytochromes: IV. Conformational homology of functionally homologous cytochromes c

Abstract

The conformational homology of functionally homologous cytochromes c from horse, beef, tuna, turkey, pigeon and moth has been investigated by comparison of the circular dichroism (CD) spectra of each preparation in both oxidation states of iron and in the wavelength region 600-185 mμ at pH 6.8. In the oxidized state all preparations exhibit similar dichroic features with only small variations in the relative magnitudes of various extrema, except for the aromatic circular dichroism curve for tuna cytochrome c. The preparations in the reduced state of iron also exhibit comparable dichroic spectra, both in the region of the Soret transition and the intrinsic absorption region. The CD patterns for the reduced cytochromes in the α-β absorption region, however, exhibit deviations suggestive of small but distinct differences in the electronic asymmetry of the prosthetic groups, especially of the porphyrin moiety, possibly because of differences in the electronically coupled chromophores. A gross conformational homology among various mammalian-type cytochromes c has, therefore, been suggested. The dichroic differences in the case of tuna cytochrome c have been imputed to be a reflection of an unusually asymmetric environment of the residue at position 33 of the polypeptide chain. The intrinsic CD curves above 200 mμ for all the preparations in both valence states of heme iron exhibit features characteristic of the presence of α-helical structures in the molecule. Below 200 mμ, however, the dichroic curves exhibit a single positive maximum at about 194 mμ and a definite minimum at 190 mμ. The former feature is usually associated with systems containing β-structure. The presence of a minimum at about 190 mμ, when taken in conjunction with the characterizing features for α-helical models (a large positive maximum at this wavelength), indicates the absence of α-helically organized structure in the molecule. An analysis of the conformation of the protein moiety in terms of proportions of reference model conformations (α-helix, β-structure and random coil) and based on the recently developed isodichroic method 24 confirms the low α-helical content of various cytochromes and further demonstrates the gross polypeptide conformational homology for various functionally homologous preparations.